Heat exchange control apparatus



Nov. 26, 1963 A. BRUNNER HEAT EXCHANGE CONTROL APPARATUS 3 Sheets-Sheet 1 Filed Sept. 12, 1960 in m a fln e r 2 4!- [E a l r m 3 Sheets-Sheet 2 Filed Sept. 12, 1960 m erlzan' A 1. FEED BIPUNNE/P.

Nov. 26, 1963 A. BRUNNER 3,111,936

HEAT EXCHANGE CONTROL APPARATUS Filed Sept. 12, 1960 3 Sheets-Sheet 3 jm eman' 141. FRED BEUNNEE.

BY fl fltl0rne 4- United States Patent 3,111,936 HEAT EXCHANGE CONTROL APPARATUS Alfred Brunner, Winterthur, Switzerland, assignor to Sulzer Freres, S.A., Winterthur, Switzerland, a corporation of Switzerland Filed Sept. 12, 1960, Ser. No. 55,530 Claims priority, application Switzerland Sept. 16, 1959 4 Claims. (Cl. 122-479) The present invention relates to an apparatus for controlling the temperatures of fluids leaving a heat exchanger wherein heat is exchanged between the fluids.

It is known to control the temperature of the reheated low pressure steam in steam power plants operating on the reheat cycle by heat exchange between the low pressure steam and high pressure steam in a heat exchanger. The temperature of the high pressure steam entering the heat exchanger is controlled by injecting water into the steam so that the reheated low pressure steam leaving the heat exchanger has a desired temperature. Instead of controlling the temperature of the reheated steam by injecting water into the high pressure steam which acts as heating medium for the low pressure steam, a by-pass may be provided for passing high pressure steam around the heat exchanger whereby the amount of by-passed high pressure steam is controlled so that the remaining high pressure steam has the desired heating effect on the low pressure steam. With the two aforesaid systems of controlling the heating effect of the high pressure steam on the low pressure steam, the temperature of the high pressure steam leaving the heat exchanger deviates from the desired temperature whenever the control comes into play because of changing load conditions, changing heat transfer conditions, and the like and additional control steps are needed for maintaining constant the temperature of the high pressure steam leaving the heat exchanger.

It is an object of the present invention to provide an apparatus for controlling the temperatures of fluids passing in heat exchange relation through an indirect heat exchanger whereby the temperatures of the heat exchanging fluids are individually maintained at a predetermined level irrespective of changing loads and conditions changing the heat exchange between the fluids. The apparatus according to the invention is particularly useful for controlling the temperatures of relatively high pressure steam and relatively low pressure steam exchanging heat in a heat exchanger which may be located Within a steam generator forming part of a steam power plant operating on the reheat cycle.

According to the invention the rate of flow of a heat exchanging fluid through a heat exchanger and the inlet temperature of a fluid passing through the heat exchanger in heat exchange relation with a second fluid are controlled in response to the temperature of at least one of the fluids leaving the heat exchanger.

An apparatus according to the invention includes means for con-trolling the temperature of a heat exchanging fluid entering a heat exchanger and means for controlling the rate of flow of a heat exchanging fluid through the heat exchanger, for example, by by-passing a controlled amount of the heat exchanging fluid per time unit around the heat exchanger, both control means being responsive to the temperature of a heat exchanging fluid as it leaves the heat exchanger.

If the apparatus according to the invention is applied to a steam power plant operating on the reheat cycle or a. steam plant in which relatively low pressure steam is reheated for other purposes, not only the temperature of resuperheated relatively low pressure steam is controlled, but also the temperature of the high pressure 3,1113% Fatented Nov. 26, 1963 'ice steam which exchanges heat with the relatively low pressure steam in a heat exchanger, is controlled.

In steam generators provided with several reheaters a heat exchanger may be associated with each reheater. The heat exchangers may be arranged in series relation or in parallel relation with respect to the flow of the high pressure steam therethrough.

The novel features which are considered characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, and additional objects and advantages thereof will best be understood from the following description of embodiments thereof when read in connection with the accompanying drawing, wherein:

FIG. 1 is a diagrammatic illustration of a control system according to the invention as applied to a steam power plant operating on the reheat cycle.

FIG. 2 is a diagrammatic illustration of a modified control system according to the invention.

FIG. 3 is a diagrammatic illustration of a modified part of the system shown in FIG. 2.

Referring more particularly to FIG. 1 of the drawing, numeral 1 designates a condensate pump pumping condensate from a condenser 19 through water heaters 4 and S to the inlet of a second condensate pump 2. The latter furnishes condensate to a. condensate heater 6 and therefrom to a boiler feedwater reservoir 9. A high pressure feed pump 3 pumps water from the reservoir 9 through 'Water heaters 7 and 3 into the evaporating tube system 10 of a steam generator. The effluent of the tube system 16 is passed through a water separator 11 and therefrom consecutively through supenheater sections 12, 13 and 14. The high pressure superheated live steam passes from the superheater section 14- into a high pressure turbine 16. The steam leaving the turbine 16 consecutively flows through a heat exchanger 17 and a reheater 1S wherefrom the low pressure steam is conducted to a low pressure turbine 18 which exhausts into the condenser 19.

The high pressure or primary side of the heat exchanger 17 receives steam from the superheater section 12 through a pipe 20 and discharges the steam through a pipe 21 into the superheater section 13. The pipes 24 and 2.1 are connected by a pipe bypassing the heat exchanger 17 and controlled by a valve 22. Water may be inieoted at 23 into the high pressure steam leaving the first superheater section 112 land at 24 into the high. pressure steam leaving the. second superheater section 13. The injection devices 23 and 24 are supplied with cooling water through pipes 25 and 26, respectively, which receive water from the feed pump 3. A valve 27 is interposed in the pipe 25 and a valve 2 8 is interposed in the pipe 26. The valves 22 and 27 are operatively connected by means of links 29 and 30 to tWO opposed arms 31 and 32 of a threeaurn lever 33. The fulcrum of the lever 33 is connected to a piston rod 34 of a hydraulic motor operator 35. The third arm 36 of the three-arm lever 33 is connected to a connecting rod 37 which is pivotally connected to a rod of a piston of a hydraulic motor operator 38. A temperature sensitive device 39 is arranged :at the inlet of the second high pressure steam superheater 13 downstream of the valve 22 and of the pipe 21. The temperature sensitive device 39 .actuates the motor operator 38 in a conventional manner by means not shown. A temperature sensitive device 4% is connected to the pipe connecting the reheater 15 and the low pressure turbine 18. The device 40 aotuates the motor operator in a conventional manner. The valve 23 is actuated by a motor operator .2 which is responsive to a temperature sensitive device 41 connected to the pipe connecting the outlet of the high pressure superheater section 14 and the high pressure turbine 16.

In the embodiment of the invention illustrated in FIG. 1, the temperature of the low pressure steam leaving the reheater is controlled by controlling the temperature as Well as the rate of flow of the high pressure steam passing through the heat exchanger 17. if, for example, the temperature of the low pressure steam leaving the rcheater 15' exceeds a predetermined value, the temperature sensitive device 49 causes movement of the piston rod 3 4extending from the motor operator to the left. This eifccts simultaneous actuation of the valves 22 and 27 so that the rates of flow of the by-passsd steam and of cooling water are increased and the rate of flow and the temperature of high pressure steam passing through the pipe 26 into the heat exchanger 17 are reduced. If the emperature of the low pressure steam leaving the reheater 15 drops below a predetermined value, the rate of fiow and the temperature of high pressure steam passing through the primary side of the heat exchanger 17 are increased by operation of the valves 22 and 27. If the temperature of the high pressure steam entering the second superhcater section 13 increases above a predetermined value, the rod 37 connecting the arm 36 of the three-arm lever 33 to the piston rod extending from the motor operator 38 moves downward. This causes clockwise movement of the three-arm lever 33 and an increase of the amount of water injected at 23 into the steam leaving the first high pressure superheater section 12 and throttling of the high pressure steam passed around the heat exchanger 17 through the valve 22. If the temperature of the steam entering the second high pressure superheater 13 drops below a predetermined value, the threearm lever 33 is turned counterclockwise whereby the amount of Water sprayed into the high pressure steam leaving the superhcater section 12 is reduced by closing the valve 27 and the amount of high pressure steam bypassed around the heat exchanger 17 through the valve 22 is increased.

By the simultaneous injection of cooling water in the high pressure steam upstream of the heat exchanger 17 and bypassing of a portion of the high pressure steam around the heat exchanger in response to the temperature of the high pressure steam, the heat made available by the high pressure steam in the heat exchanger is controlled independently of the requirement of the heat exchanger. Since in the illustrated embodiment of the invention the high pressure steam is cooled by the low pressure steam in the heat exchanger 17, the simultaneous increase of the amount of water injected at 23 and of the water by-passed around the heat exchanger through the valve 22 can be so proportioned that these control steps balance each other and the resulting temperature of the high pressure steam remains unchanged. Undesired changes of the temperature of the high pressure steam entering the superheater section 13 are prevented by actuation of the valves 22 and 27 in opposite directions by the motor operator 33.

FIG. 2 illustnates a modified system of the control of a heat exchanger forming part of a system for superheating high pressure steam and resnperheating relatively low pressure steam in a steam power plant operating on the reheat cycle. FIG. 2 illustrates only the superheating sections including water injection devices of a steam generator forming part of the steam power plant. The steam generator is indicated by d=ash-dotted lines in FIG. 2. The high pressure steam superheater comprises tubular heating sections 101, 102 and 163 arranged in series relation with respect to the flow of the high pressure steam. A reheater 104 for relatively low pressure steam is heated by the same products of combustion as the high pressure steam superheater. The steam leaving the final section 103 of the high pressure superheater is conducted through a pipe 105 to the high pressure part of a turbine, not shown. The reheater 104 is connected to the low pressure part of the turbine by means of a pipe 186. The steam exhausted by the high pressure part of the turbine is conducted through a pipe 107 to the outer tube 108 of a heat exchanger 110 and is conducted from the tube 108 through a pipe 111 to the reheater 104. The heat exchanger 110 may be placed in the flue of the steam generator to receive heat from the gases of combustion, or may be placed in the combustion chamber of the steam generator to receive radiant heat. One or more high pressure tubes 199 may be placed inside the low pressure tube 103 and connected in parallel relation with respect to the flow of the low pressure steam, only one tube being shown in order to simplify the diagrammatic illustration. The tube 169 is interposed in the pipe line connecting the first high pressure superheater section 101 and the second high pressure superhcater section 102. A by-pass line 112 by-passes high pressure steam around the high pressure pipe 109 in the heat exchanger 110. The by-pass line 112 is controlled by means of a valve or flow control means 113. Water injection into the high pressure steam is effected upstream of the heat cxchanger by mcans of a device 114 and between the superheater sections 102 and 103 by means of a device 115. The devices 114 and 115 are connected by pipes 116 and 117, respectively, to the outlet of a high pressure feed pump, not shown. A valve 118 is interposed in the pipe 116 and a valve 119 is interposed in the pipe 117. The pipe 106 is provided with a temperature sensitive device 120 which will be described later and which actuates a hydraulic control signal producer 121 which also will be described later. The high pressure pipe interconnecting the pipe 109 and the second high pressure steam superheater 102 is provided with a temperature sensitive device 122 which actuates a control signal producer 123.

The control signals produced by the device 121 are conducted through a conduit 124 to a cylinder 125 containing a piston valve 126, springs 127 being provided in order to hold the valve 126 in a middle position. The valve 126 controls flow or" a pressure fluid through pipes 129 provided with throttling means 128 to and from a cylinder 139. The piston valve 126 divides the inside of the cylinder 125 into two chambers the upper one of which is connected to the conduit 124 and the lower one of which is connected by means of a conduit 131 to a hand-operated set point signal producing device 132. Rotation of a handwheel 134 provided with a threaded spindle 133 rotating in a threaded bore of a. stationary element and acting through a spring on a piston in the device 132 produces a desired signal pressure in the device 132 and in the pipe 131.

The cylinder has an upper chamber containing a piston 135 provided with a piston rod 136 extending into a lower chamber in the cylinder 130 and acting on a spring 299 which abuts against a piston 207 in the lower part 292 of the cylinder 130, forming a signabproducing device 137. The chamber above the piston 207 is connected to the conduit 124 by means of a pipe 138.

The signal pressure produced in the device 137 is conducted 11hr 211 a pipe 139 to the upper part of a cylinder 140 containing a piston 14-1 and a spring pressing the piston against the pressure in the upper part of the cylin- The Piston 141 is provided on each side with The flow control means 113 is connected by means of a piston rod 145 to a piston 146 in a cylinder 147. The piston 146 is yieldingly held in a middle position by means of springs. The space in the cylinder 147 at the right side of the piston 146 is connected by means of a pipe 148 to the chamber above the piston 14 1 in the is, therefore, also connected to the control signal producer 137. The chamber in the cylinder The pipe 149 is connected through a pipe 14% to the chamber in the cylinder 144 above the piston 143 so that the last mentioned chamber is also connected. to the signal producer 152. Each of the pipes 148 and 149 is provided with a throttling element 14%. The lower chambers in the cylinders 14% and M4 are connected to the atmosphere by means of apertures 150.

The signal producer 1521 in not only actuated on by the pressure fluid in the pipe 149 but also by a piston 154 in a cylinder 153. The chambers in the cylinder 153 above and below the piston 154 are connected through pipes 155 provided with throttling elements 156 to a piston valve 157. The latter controls supply of pressure fluid to and relief of pressure fluid from the cylinder 153 and is actuated upon by springs 158 and 159. The force of the spring 159 which opposes the [force of the spring 158 may be controlled by revolving a threaded spindle 161} in a corresponding threaded stationary bore by manipulating a hand wheel 161. The spring 158 is connected by means of a rod 162 to the valve or flow control rneans 119 whereby the force of the spring 158 which counteracts the spring 15? is adjusted in response to the position or opening of the valve 119 The latter is contnolled in a conventional manner in response to the temperature of the high pressure steam in the steam main 1115, a tempenature sensitive device 163 actuating a motor operator 164 for the valve 119 being provided for this purpose. The motor operator 164 preferably has a proportional integral characteristic.

The hydraulic signal producers 1'23, 121, 132, 137, 152 shown in FIG. 2 individually include .a cup-shaped piston 201 movable in a cylinder 2G2 and loaded at one side by a spring 2%. The piston 2111 is provided with an aperture 2'03 con-trolling flow of pnessure fluid into and out of the cylinder 2111 through ports "204 and 295. If, for example, the piston 201 of the signal producer 121 is moved to the night as seen in FIG. 2, the port 205 is opened and pressure fluid is admitted to the cylinder 2112 so that the pressure in the cylinder "and pipe connected thereto is increased. If the piston 2111 of the device 121 is moved to the left, the port 2114 is opened and pressure fluid is relieved from the cylinder 26 2 so that the pressure in the cylinder and pipe connected thereto is reduced. The pressure acting on the piston 2111 is counterbalanced by the spring 260 and the signal pressure in the line 124 is proportional to the pressure exerted by the spring 200.

The temperature sensitive devices 121) and 122 shown in FIG. 1 include a rodlike element arranged parallel to the pipe whose temperature must be measured and having a heat expansion coeflicient which is substantially smaller than that of the respective pipe. The latter expands in response to the temperature or the steam passing through the pipe. The relative expansion of the rodlike element and the steam pipe in connection with which it is used is amplified by a lever having an end acting on the spring 2% of a control signal producer. In the arrangements shown in FIG. 2 the temperature sensitive devices 121) and 12 2 compress the springs 2119 upon an increase of the temperature of the respective steam pipes.

If the temperature of the tube 1% increases, the temperature sensitive device 1213 increases the spring pressure acting on the piston 201 of the signal producing device 121 so that the pressure in the pipe 124- and also in the pipe 138 connected thereto increases. This causes an increase of the pressure acting on the piston 2111 of the signal producer 13-7 so that also the pressure in the pipes 139 and 148 is increased. This causes a downward movement of the piston 141 and a movement to the left of the piston 146. As a result the openings of the valves 113 and 118 are increased. Since, due to the excessive temperature of the low pressure steam in the pipe 196, the pressure in the pipe 124 is greater than the pressure in the pipe 131 which is controlled by the signal producer 132, the piston valve 126 is moved downward and admits pressure fluid into the chamber of the cylinder 131} above the piston 135 and relieves pressure fluid from the chamber below the piston 135. This causes downward movement of the piston 135, which is relatively slow due to the provision of the throttling elements 128, and also relatively slow downward movement of the piston 261 of the signal producer 137. The effect of the piston valve 126 on the device 137 remains until the temperature of the pipe 106 is returned to the temperature defined by the signal producer or set point controller 132 whereupon the pressures in the pipes 124 and 131 are balanced.

The temperature sensitive device 122 actuates, through a signal producer 123, the signal producer 152 upon a change of the temperature of the steam entering the second high pressure steam superheater section 102. The signal producer 152 changes the pressure in the cyiinder 147 at the left side 01 the piston 146 and also in the upper chamber of the cylinder 144 so that the flow control means 113 and 118 are actuated in the opposite sense.

The signal producer 152 is also affected by' the movement of the piston 154.- which is controlled by the piston valve 157 if the rate of flow of cooling water through the valve 119 to the injector does not correspond to the rate of flow set by the set point adjusting device 1611, 161. This supplemental adjustment of the signal producer 152 causes a supplemental adjustment of the flow control means 113 and 11% resulting in a temperature of the high pressure steam requiring a rate of flow of cooling water to the injecting device 115 which rate of flow corresponds to the rate of flow prescribed by the set point adjusting device 1611, 16-1. The supplemental adjustment is delayed by the throttling elements 156.

The throttling elements 14% interposed in the pipes 148 and 149 cause a delay of actuation of the flow control device 113. Since the by-pas-s conduit 1-12 is shorter than the path of the high pressure steam through the heat exchanger 111 a change of opening of the valve 113 affects the temperature of the high pressure steam quicker than a change of the rate of flow of cooling water into the high pressure steam. The throttling elements 149]) delay the actuation of the valve 113 relative to the actuation of the valve 118 so that a change of the rate of flow of high pressure steam through the bypass conduit 112 is eifected substantially simultaneously with a change of cooling water supply to the injector 114. This stabilizes the regulating process.

FIG. 3 shows a modification of a part of the system illustrated in FIG. 2. In the arrangement according to FIG. 3 the heat exchanger 111i is provided with a bypass conduit 3110 by-passing the low pressure tube 108. A flow control device 301 is interposed in the by-pass conduit 3% and is connected by a rod 3113 to a piston 3112 in a cylinder 3%- of a hydraulic motor operator. The piston 3112 is placed between springs tending to hold the piston in a middle position. The chambers in the cylinder 304 at the sides of the piston 302 are connected by pipes 148 and 149' to the control signal producers 137 and 152. as shown in FIG. 2.

In the embodiment shown in FIG. 3 the two control steps, namely, the temperature control and the rate of flow control are performed on two different heat exchanging fluids in co-ntradistinction to the modifications shown in F163. 1 and 2 wherein the two control steps are performed on the same heat exchanging fluid. The effect of the system shown in P16. 3 is the same as that of the systems shown in FIGS. 1 and 2.

The invention can not only be applied to heat exchangers combined with steam generators of the described and illustrated type, but can be applied to heat exchangers used in different combinations without departing from the scope of the invention. The temperature of one of the heat exchanging fluids entering the heat exchanger need not be controlled by cooling, for example injection cooling as described, but may be controlled by heating. The invention is not limited to a system in which the temperature of the hotter or heating fluid is controlled but can also be modified to control the temperature of the cooler fluid which is heated by the hotter fluid and thereby cools the latter. Depending on the combination in which the fundamental idea of the invention is used, the two control elements one of which controls the rate of flow of heat exchanging fluid through the heat exchanger and the other of which controls the temperature of a heat exchanging fluid entering the heat exchanger are actuated in the same sense upon a change of the temperature of one of the heat exchanging fluids as it leaves the heat exchanger and are actuated in the opposite sense upon a change of the temperature of the other heat exchanging fluid leaving the heat exchanger.

The system according to the invention may be simplified by making the control responsive to the temperature of only one of the heat exchanging fluids leaving the heat exchanger. Referring more particularly to FIG. 1, the temperature sensitive device 39 and the motor operator 3 8 may be omitted. In this case the three-arm lever 3-3 is replaced by rigid connection of the rods 29 and 3%. This simplified system does not avoid certain deviations of the temperature of the high pressure steam leaving the heat exchanger; these deviations, however, are substantially smaller than the temperature fluctuations experienced in conventional systems.

Without departing from the spirit and scope of the invention as claimed, a heat exchanger provided with temperature controls according to the invention may be supplied with partly expanded steam, serving, instead or live steam, as the relatively high pressure steam which exchanges heat with steam of still lower pressure and to be reheated.

I claim:

1. An apparatus for controlling a heat exchanger exchanging heat between two fluids, comprising a first conduit for conducting a first fluid, a second conduit for conducting a second fluid, a heat exchanger interposed in said conduits wherein said conduits are placed in heat exchange relation, a by-pass pipe connected to one of said conduits upstream and downstream of said heat exchanger for by-passing fluid flowing in said last mentioned conduit around said heat exchanger, a valve in said by-pass pipe, adjustable cooling means connected to one of said conduits upstream of said heat exchanger for cooling the fluid entering said heat exchanger, regulating means connected to said cooling means for regulating the cooling effect thereof, a temperature sensing means connected to each of said conduits downstream of said heat exchanger, a first of said temperature sensing means being operatively connected to said valve and to said regulating means for opening said valve for decreasing the rate of flow of the respective fluid through said heat exchanger and for simultaneously and interdependently increasing the cooling effect of said cooling means upon an increase of the temperature sensed by said first sensing means above a predetermined value and conversely, the second sensing means being openatively connected to said valve and to said regulating means for closing said valve for increasing the rate of flow of the respective fluid through said heat exchanger and for simultaneously and interdependently increasing the cooling effect of said cooling means upon an increase of the temperature sensed by said second sensing means above a predetermined value and conversely.

2. In a steam power plant operating on the reheat cycle and under variable load conditions, a high pressure tube system including a steam generator having duct means for passing hot products of combustion, a relatively low pressure tube system including a resuperheater placed in said duct means for reheating relatively low pressure steam, a heat exchanger interposed in said tube systems for exchanging heat between high pressure steam flowing in said high pressure tube system and low pressure steam flowing in said low pressure tube system, a by-pass pipe connected to said high pressure tube system upstream and downstream of said heat exchanger for by-passing high pressure steam therearound, a valve in said by-pass pipe, a water supply pipe connected to said high pressure tube system upstream of said heat exchanger for injecting water into said high pressure tube system, a valve in said water supply pipe, a first temperature sensing means connected to said low pressure tube system downstream of said heat exchanger, 2. second temperature sensing means connected to said high pressure tube system downstream of said heat exchanger, said first temperature sensing means being operativley connected to both of said valves for opening said valves for decreasing the rate of flow of high pressure steam through said heat exchanger and for simultaneously and interdependently increasing water injection upon an increase of the temperature of the low pressure steam above a predetermined value and conversely, the second temperature sensing ".eans being operatively connected to both said valves for simultaneously closing said valve in said by-pass pipe for increasing the rate of flow of high pressure steam through said heat exchanger and for simultaneously and interdependently opening said valve in said water supply pipe for increasing water injection upon an increase of the temperature of the high pressure steam above a predetermined value and conversely.

3. In a steam power plant as defined in claim 2, actuating means operatively connected to both of said valves and to both of said temperature sensing means for actuation thereby and adapted to actuate both valves in the same sense upon actuation of said actuating means by said first temperature sensing means and to actuate said valves in opposite senses with respect to each other upon actuation of said actuating means by said second temperature sensing means.

4. In a steam power plant as defined in claim 2 and wherein said heat exchanger is placed in said duct means.

References Cited in the file of this patent UNITED STATES PATENTS 2,848,983 Lieberherr Aug. 26, 1958 2,852,005 Buri Sept. 16, 1958 2,966,896 Vogler Jan. 3, 1961 FOREIGN PATENTS 765,140 Great Britain Jan. 2, 1957 

1. AN APPARATUS FOR CONTROLLING A HEAT EXCHANGER EXCHANGING HEAT BETWEEN TWO FLUIDS, COMPRISING A FIRST CONDUIT FOR CONDUCTING A FIRST FLUID, A SECOND CONDUIT FOR CONDUCTING A SECOND FLUID, A HEAT EXCHANGER INTERPOSED IN SAID CONDUITS WHEREIN SAID CONDUITS ARE PLACED IN HEAT EXCHANGE RELATION, A BY-PASS PIPE CONNECTED TO ONE OF SAID CONDUITS UPSTREAM AND DOWNSTREAM OF SAID HEAT EXCHANGER FOR BY-PASSING FLUID FLOWING IN SAID LAST MENTIONED CONDUIT AROUND SAID HEAT EXCHANGER, A VALVE IN SAID BY-PASS PIPE, ADJUSTABLE COOLING MEANS CONNECTED TO ONE OF SAID CONDUITS UPSTREAM OF SAID HEAT EXCHANGER FOR COOLING THE FLUID ENTERING SAID HEAT EXCHANGER, REGULATING MEANS CONNECTED TO SAID COOLING MEANS FOR REGULATING THE COOLING EFFECT THEREOF, A TEMPERATURE SENSING MEANS CONNECTED TO EACH OF SAID CONDUITS DOWNSTREAM OF SAID HEAT EXCHANGER, A FIRST OF SAID TEMPERATURE SENSING MEANS BEING OPERATIVELY CONNECTED TO SAID VALVE AND TO SAID REGULATING MEANS FOR OPENING SAID VALVE FOR DECREASING THE RATE OF FLOW OF THE RESPECTIVE FLUID THROUGH SAID HEAT EXCHANGER AND FOR SIMULTANEOUSLY AND INTERDEPENDENTLY INCREASING THE COOLING EFFECT OF SAID COOLING MEANS UPON AN INCREASE OF THE TEMPERATURE SENSED BY SAID FIRST SENSING MEANS ABOVE A PREDETERMINED VALUE AND CONVERSELY, THE SECOND SENSING MEANS BEING OPERATIVELY CONNECTED TO SAID VALVE AND TO SAID REGULATING MEANS FOR CLOSING SAID VALVE FOR INCREASING THE RATE OF FLOW OF THE RESPECTIVE FLUID THROUGH SAID HEAT EXCHANGER AND FOR SIMULTANEOUSLY AND INTERDEPENDENTLY INCREASING THE COOLING EFFECT OF SAID COOLING MEANS UPON AN INCREASE OF THE TEMPERATURE SENSED BY SAID SECOND SENSING MEANS ABOVE A PREDETERMINED VALUE AND CONVERSELY. 